Adenovirus E4-ORF3-dependent relocalization of TIF1α and TIF1γ relies on access to the Coiled-Coil motif

Adenovirus Remodeling of the Host Proteome and Host Factors Associated with Viral Genomes

Viral infections are associated with extensive remodeling of the cellular proteome. Viruses encode gene products that manipulate host proteins to redirect cellular processes or subvert antiviral immune responses. Adenovirus (AdV) encodes proteins from the early E4 region which are necessary for productive infection. Some cellular antiviral proteins are known to be targeted by AdV E4 gene products, resulting in their degradation or mislocalization. However, the full repertoire of host proteome changes induced by viral E4 proteins has not been defined. To identify cellular proteins and processes manipulated by viral products, we developed a global, unbiased proteomics approach to analyze changes to the host proteome during infection with adenovirus serotype 5 (Ad5) virus. We used whole-cell proteomics to measure total protein abundances in the proteome during Ad5 infection. Since host antiviral proteins can antagonize viral infection by associating with viral genomes and inhibiting essential viral processes, we used Isolation of Proteins on Nascent DNA (iPOND) proteomics to identify proteins associated with viral genomes during infection with wild-type Ad5 or an E4 mutant virus. By integrating these proteomics data sets, we identified cellular factors that are degraded in an E4-dependent manner or are associated with the viral genome in the absence of E4 proteins. We further show that some identified proteins exert inhibitory effects on Ad5 infection. Our systems-level analysis reveals cellular processes that are manipulated during Ad5 infection and points to host factors counteracted by early viral proteins as they remodel the host proteome to promote efficient infection. IMPORTANCE Viral infections induce myriad changes to the host cell proteome. As viruses harness cellular processes and counteract host defenses, they impact abundance, post-translational modifications, interactions, or localization of cellular proteins. Elucidating the dynamic changes to the cellular proteome during viral replication is integral to understanding how virus-host interactions influence the outcome of infection. Adenovirus encodes early gene products from the E4 genomic region that are known to alter host response pathways and promote replication, but the full extent of proteome modifications they mediate is not known. We used an integrated proteomics approach to quantitate protein abundance and protein associations with viral DNA during virus infection. Systems-level analysis identifies cellular proteins and processes impacted in an E4-dependent manner, suggesting ways that adenovirus counteracts potentially inhibitory host defenses. This study provides a global view of adenovirus-mediated proteome remodeling, which can serve as a model to investigate virus-host interactions of DNA viruses.

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

Adenoviral strategies to overcome innate cellular responses to infection

Viruses alter host cell processes to optimize their replication cycle. Human adenoviruses (Ad) encode proteins that promote viral macromolecular synthesis and counteract innate and adaptive responses to infection. The focus of this review is on how Ad evades innate cellular responses to infection, including an interferon (IFN) response and a DNA damage response (DDR). Ad blocks the IFN response by inhibiting cytoplasmic signaling pathways and the activation of IFN-stimulated genes (ISGs), as well as the functions of ISG products, such as PML. Ad also inhibits DDR sensors, for instance, the Mre11-Rad50-Nbs1 complex, and DDR effectors like DNA ligase IV. These innate cellular responses impact many different viruses, and studies on Ad have provided broad insight into these areas.

Mechanism of Adenovirus E4-ORF3-Mediated SUMO Modifications

Regulation of a variety of different cellular processes, including posttranslational modifications, is critical for the ability of many viruses to replicate efficiently within host cells. The adenovirus (Ad) E4-ORF3 protein assembles into polymers and forms a unique nuclear scaffold that leads to the relocalization and sequestration of cellular proteins, including small ubiquitin-like modifiers (SUMOs). Previously, we showed that E4-ORF3 functions as a SUMO E3 ligase of transcriptional intermediary factor-1 gamma (TIF-1γ) and promotes poly-SUMO chain formation. Here, we present cellular and biochemical data to further understand E4-ORF3 SUMO ligase activity. E4-ORF3 proteins from five different Ad species were found to possess SUMO E3 ligase activities in vitro In infected cells, SUMO modifications of target proteins occurred only when the proteins were recruited into E4-ORF3 polymeric structures. By analyzing SUMO-deficient TIF-1γ, we demonstrated that SUMO conjugations are not required for E4-ORF3-mediated relocalization of target proteins in infected cells, implying that sequestration is followed by SUMO modification. In vitro SUMO conjugation assays revealed the Ad E1B-55K oncoprotein as a new viral target of E4-ORF3-mediated SUMOylation. We also verified a direct function of E4-ORF3 as a SUMO ligase for multiple cellular proteins, including transcription factor II-I (TFII-I), Nbs1, and Mre11. Moreover, we discovered that E4-ORF3 associates with SUMO-bound UBC9, and E4-ORF3 polymerization is crucial for this ternary interaction. Together, our findings characterize E4-ORF3 as a novel polymer-type SUMO E3 ligase and provide mechanistic insights into the role of E4-ORF3 in SUMO conjugation.IMPORTANCE Viruses interplay with the host SUMOylation system to manipulate diverse cellular responses. The Ad E4-ORF3 protein forms a dynamic nuclear network to interfere with and exploit different host processes, including the DNA damage and interferon responses. We previously reported that E4-ORF3 is a SUMO E3 ligase. Here, we demonstrate that this activity is a conserved function of evolutionarily diverse human Ad E4-ORF3 proteins and that E4-ORF3 functions directly to promote SUMO conjugations to multiple cellular proteins. Recruitment of cellular substrates into E4-ORF3 nuclear inclusions is required for SUMO conjugation to occur in vivo We probed the mechanism by which E4-ORF3 functions as a SUMO E3 ligase. Only multimeric, but not dimeric, E4-ORF3 binds to the SUMO E2 conjugation enzyme UBC9 in vitro only in a trimeric complex with SUMO. These results reveal a novel mechanism by which a conserved viral protein usurps the cellular SUMO conjugation machinery.

Targeting Sirt1 in a rat model of high-fat diet-induced non-alcoholic fatty liver disease: Comparison of Gegen Qinlian decoction and resveratrol

The present study aimed to explore the mechanism of action of Gegen Qinlian decoction (GGQLD) in experimental non-alcoholic fatty liver disease (NAFLD). A total of 30 rats were randomly divided into five groups: The chow, model, high- and low-dose GGQLD (GGQLD-H and GGQLD-L, respectively) and resveratrol (Resl) groups, and were treated with saline, GGQLD and Resl when a model of high-fat diet (HFD)-induced NAFLD was established. Blood lipid and liver enzymes were detected following treatment for 8 weeks and liver tissue pathology was observed using Oil Red O and haematoxylin and eosin staining. Furthermore, the liver protein and mRNA expression of sirtuin (Sirt)1, peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α and nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) were measured using western blotting and reverse transcription-quantitative polymerase chain reaction. Compared with the chow group, the model group demonstrated significantly increased serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels (P<0.01). GGQLD doses and Resl attenuated the elevated serum ALT and AST levels. GGQLD-H and Resl significantly increased the serum high-density lipoprotein cholesterol level compared with that in the model group (P<0.01), while GGQLD-L and Resl significantly decreased serum low-density lipoprotein cholesterol levels (P<0.01). The GGQLDs and Resl groups revealed an evident improvement in Sirt1 protein and mRNA expression. Although GGQLD and Resl significantly decreased NF-κB gene expression compared with the model group (P<0.01), the effect on NF-κB protein expression was not significant. Furthermore, the PGC-1α gene and protein expression in the HFD rat group slightly decreased compared to the levels in the chow group, but the decrease was insignificant. However, an evident increase in PGC-1α mRNA expression was observed in the GGQLD-H group compared with the model group (P<0.01). Histological staining revealed that GGQLD and Resl decreased the lipid droplets in hepatocytes and normalized steatosis in rats fed with a HFD. The results indicated that GGQLD treatment may be a potent strategy for managing NAFLD by managing lipid metabolism and inflammatory and histological abnormalities by triggering the Sirt1 pathway.

Effects of indigo naturalis on colonic mucosal injuries and inflammation in rats with dextran sodium sulphate-induced ulcerative colitis

The effects of indigo naturalis (IN), which is a traditional Chinese herbal formulation, have been clinically demonstrated in treating refractory ulcerative colitis (UC). The present study aimed to verify the effects and mechanisms of IN in experimental UC rats. A total of 48 male Sprague-Dawley rats were randomly divided into six groups: Chow, model, high-dose IN, medium-dose IN, low-dose IN and mesalazine (a bowel-specific aminosalicylate drug) groups. The models were administered 3.5% dextran sodium sulphate solution for 7 days. The treatment groups were administered IN or mesalazine and then sacrificed and sampled on day 8. Disease activity index (DAI), histological damage score (HDS) and myeloperoxidase (MPO) activity were used to evaluate the severity of UC. Colon and serum cytokines were detected using liquid-phase chip technology and the expression of occludin protein in colonic mucosa was assessed by immunohistochemistry and western blot analysis. The results indicated that the oral administration of IN may reduce DAI, HDS and MPO activity. IN also reduced the expression of inflammatory cytokines and increased the expression of colonic mucosal repair-related cytokines and occludin protein. These results highlight the potential of IN as a therapeutic agent for treating UC through its action of inflammation control and colonic mucosal damage repair.

Adenovirus Early Proteins and Host Sumoylation

The human adenovirus genome is transported into the nucleus, where viral gene transcription, viral DNA replication, and virion assembly take place. Posttranslational modifications by small ubiquitin-like modifiers (SUMOs) are implicated in the regulation of diverse cellular processes, particularly nuclear events. It is not surprising, therefore, that adenovirus modulates and utilizes the host sumoylation system. Adenovirus early proteins play an important role in establishing optimal host environments for virus replication within infected cells by stimulating the cell cycle and counteracting host antiviral defenses. Here, we review findings on the mechanisms and functional consequences of the interplay between human adenovirus early proteins and the host sumoylation system.

The adenovirus E4-ORF3 protein functions as a SUMO E3 ligase for TIF-1γ sumoylation and poly-SUMO chain elongation

The adenovirus (Ad) early region 4 (E4)-ORF3 protein regulates diverse cellular processes to optimize the host environment for the establishment of Ad replication. E4-ORF3 self-assembles into multimers to form a nuclear scaffold in infected cells and creates distinct binding interfaces for different cellular target proteins. Previous studies have shown that the Ad5 E4-ORF3 protein induces sumoylation of multiple cellular proteins and subsequent proteasomal degradation of some of them, but the detailed mechanism of E4-ORF3 function remained unknown. Here, we investigate the role of E4-ORF3 in the sumoylation process by using transcription intermediary factor (TIF)-1γ as a substrate. Remarkably, we discovered that purified E4-ORF3 protein stimulates TIF-1γ sumoylation in vitro, demonstrating that E4-ORF3 acts as a small ubiquitin-like modifier (SUMO) E3 ligase. Furthermore, E4-ORF3 significantly increases poly-SUMO3 chain formation in vitro in the absence of substrate, showing that E4-ORF3 has SUMO E4 elongase activity. An E4-ORF3 mutant, which is defective in protein multimerization, exhibited severely decreased activity, demonstrating that E4-ORF3 self-assembly is required for these activities. Using a SUMO3 mutant, K11R, we found that E4-ORF3 facilitates the initial acceptor SUMO3 conjugation to TIF-1γ as well as poly-SUMO chain elongation. The E4-ORF3 protein displays no SUMO-targeted ubiquitin ligase activity in our assay system. These studies reveal the mechanism by which E4-ORF3 targets specific cellular proteins for sumoylation and proteasomal degradation and provide significant insight into how a small viral protein can play a role as a SUMO E3 ligase and E4-like SUMO elongase to impact a variety of cellular responses.

Impact of Adenovirus E4-ORF3 Oligomerization and Protein Localization on Cellular Gene Expression

The Adenovirus E4-ORF3 protein facilitates virus replication through the relocalization of cellular proteins into nuclear inclusions termed tracks. This sequestration event disrupts antiviral properties associated with target proteins. Relocalization of Mre11-Rad50-Nbs1 proteins prevents the DNA damage response from inhibiting Ad replication. Relocalization of PML and Daxx impedes the interferon-mediated antiviral response. Several E4-ORF3 targets regulate gene expression, linking E4-ORF3 to transcriptional control. Furthermore, E4-ORF3 was shown to promote the formation of heterochromatin, down-regulating p53-dependent gene expression. Here, we characterize how E4-ORF3 alters cellular gene expression. Using an inducible, E4-ORF3-expressing cell line, we performed microarray experiments to highlight cellular gene expression changes influenced by E4-ORF3 expression, identifying over four hundred target genes. Enrichment analysis of these genes suggests that E4-ORF3 influences factors involved in signal transduction and cellular defense, among others. The expression of mutant E4-ORF3 proteins revealed that nuclear track formation is necessary to induce these expression changes. Through the generation of knockdown cells, we demonstrate that the observed expression changes may be independent of Daxx and TRIM33 suggesting that an additional factor(s) may be responsible. The ability of E4-ORF3 to manipulate cellular gene expression through the sequestration of cellular proteins implicates a novel role for E4-ORF3 in transcriptional regulation.

Proteomic analysis of ubiquitin-like posttranslational modifications induced by the adenovirus E4-ORF3 protein

Viruses interact with and regulate many host metabolic pathways in order to advance the viral life cycle and counteract intrinsic and extrinsic antiviral responses. The human adenovirus (Ad) early protein E4-ORF3 forms a unique scaffold throughout the nuclei of infected cells and inhibits multiple antiviral defenses, including a DNA damage response (DDR) and an interferon response. We previously reported that the Ad5 E4-ORF3 protein induces sumoylation of Mre11 and Nbs1, which are essential for the DDR, and their relocalization into E4-ORF3-induced nuclear inclusions is required for this modification to occur. In this study, we sought to analyze a global change in ubiquitin-like (Ubl) modifications, with particular focus on SUMO3, by the Ad5 E4-ORF3 protein and to identify new substrates with these modifications. By a comparative proteome-wide approach utilizing immunoprecipitation/mass spectrometry, we found that Ubl modifications of 166 statistically significant lysine sites in 51 proteins are affected by E4-ORF3, and the proteome of modifications spans a diverse range of cellular functions. Ubl modifications of 92% of these identified sites were increased by E4-ORF3. We further analyzed SUMO3 conjugation of several identified proteins. Our findings demonstrated a role for the Ad5 E4-ORF3 protein as a regulator of Ubl modifications and revealed new SUMO3 substrates induced by E4-ORF3.

IMPORTANCE

The adenovirus E4-ORF3 protein induces dynamic structural changes in the nuclei of infected cells and counteracts host antiviral responses. One of the mechanisms that accounts for this process is the relocalization and sequestration of cellular proteins into an E4-ORF3 nuclear scaffold, but little is known about how this small viral protein affects diverse cellular responses. In this study, we analyzed for the first time the global pattern of ubiquitin-like (Ubl) modifications, with particular focus on SUMO3, altered by E4-ORF3 expression. The results suggest a role for the Ad5 E4-ORF3 protein as a regulator of Ubl modifications and reveal new SUMO3 substrates targeted by E4-ORF3. Our findings propose Ubl modifications as a new mechanism by which E4-ORF3 may modulate cellular protein functions in addition to subnuclear relocalization.

Biophysical and functional analyses suggest that adenovirus E4-ORF3 protein requires higher-order multimerization to function against promyelocytic leukemia protein nuclear bodies

The early region 4 open reading frame 3 protein (E4-ORF3; UniProt ID P04489) is the most highly conserved of all adenovirus-encoded gene products at the amino acid level. A conserved attribute of the E4-ORF3 proteins of different human adenoviruses is the ability to disrupt PML nuclear bodies from their normally punctate appearance into heterogeneous filamentous structures. This E4-ORF3 activity correlates with the inhibition of PML-mediated antiviral activity. The mechanism of E4-ORF3-mediated reorganization of PML nuclear bodies is unknown. Biophysical analysis of the purified WT E4-ORF3 protein revealed an ordered secondary/tertiary structure and the ability to form heterogeneous higher-order multimers in solution. Importantly, a nonfunctional E4-ORF3 mutant protein, L103A, forms a stable dimer with WT secondary structure content. Because the L103A mutant is incapable of PML reorganization, this result suggests that higher-order multimerization of E4-ORF3 may be required for the activity of the protein. In support of this hypothesis, we demonstrate that the E4-ORF3 L103A mutant protein acts as a dominant-negative effector when coexpressed with the WT E4-ORF3 in mammalian cells. It prevents WT E4-ORF3-mediated PML track formation presumably by binding to the WT protein and inhibiting the formation of higher-order multimers. In vitro protein binding studies support this conclusion as demonstrated by copurification of coexpressed WT and L103A proteins in Escherichia coli and coimmunoprecipitation of WT·L103A E4-ORF3 complexes in mammalian cells. These results provide new insight into the properties of the Ad E4-ORF3 protein and suggest that higher-order protein multimerization is essential for E4-ORF3 activity.